1. Field of the Invention
The present invention is directed in general to biological enzyme-based fuel cells (a.k.a. biofuel cells) and their methods of manufacture and use. More specifically, the invention is directed to biocathodes and their method of manufacture and use.
2. Description of Related Art
References cited throughout this specification are incorporated herein by reference. The discussion of those references is intended merely to summarize the assertions made by their authors and no admission is made that any reference constitutes prior art. Applicants reserve the right to challenge the accuracy and pertinence of the cited references.
A biofuel cell is an electrochemical device in which energy derived from chemical reactions is converted to electrical energy by means of the catalytic activity of living cells and/or their enzymes. Biofuel cells generally use complex molecules to generate at the anode the hydrogen ions required to reduce oxygen to water, while generating free electrons for use in electrical applications. A biocathode is the electrode of the biofuel cell where electrons and protons from the anode are used by the catalyst to reduce oxygen to water. A biofuel cell is similar to a traditional polymer electrolyte membrane (“PEM”) fuel cell in that it consists of a cathode and anode separated by some sort of barrier or salt bridge, such as for example a polymer electrolyte membrane. Biofuel cells differ from the traditional fuel cell by the material used to catalyze the electrochemical reaction. Rather than using precious metals as catalysts, biofuel cells rely on biological molecules such as enzymes to carry out the reaction. Although early biofuel cell technology employed metabolic pathways of whole microorganisms, the problems associated with this approach include low volumetric catalytic activity of the whole organism and impractical power density outputs [Palmore and Whitesides, 1994, ACS Symposium Series 566:271-290]. Enzyme isolation techniques spurred advancement in biofuel cell applications by increasing volumetric activity and catalytic capacity [Palmore and Whitesides, 1994, ACS Symposium Series 566:271-290]. Isolated enzyme biofuel cells yield increased power density output by overcoming interferences associated with cellular membrane impedance with electron transfer and lack of fuel consuming microbial growth.
Although enzymes are highly efficient catalysts, there have been problems incorporating them into fuel cells. Early enzyme-based fuel cells contained enzymes in solution rather than immobilized on the electrode surface [Palmore and Whitesides, 1994, ACS Symposium Series 566:271-290 and references within, which are incorporated herein by reference]. Enzymes in solutions are only stable for days, whereas immobilized enzymes can be stable for months. One of the main obstacles of enzyme-based biofuel cells has been to immobilize the enzyme in a membrane at the electrode surface that will extend the lifetime of the enzyme and form a mechanically and chemically stable layer, while not forming a capacitive region at the electrode surface. In most H2/O2 fuel cells, the binder that holds the catalyst at the electrode surface is Nafion®. Nafion® is a perfluorinated ion exchange polymer that has excellent properties as an ion conductor. However, Nafion® has not been successful at immobilizing enzymes at the surface of biofuel cell electrodes because Nafion® forms an acidic membrane that decreases the lifetime and activity of the enzyme.
Several attempts have been made by others to develop biofuel cells that incorporate immobilized enzymes. Various methods of immobilizing enzymes for use in biological fuel cells, wherein the enzymes show at least, minimal activity and stability are described in U.S. Pat. No. 6,294,291, U.S. Pat. No. 6,531,239 and Chen et al., J. Am. Chem. Soc. 2001, vol. 123:8630-8631, which are incorporated herein by reference. Those references describe the immobilization of various redox enzymes (oxidoreductases) onto polymer sol gel matrices, which also incorporate electron transfer mediators, such as osmium, cobalt or ruthenium complexes. However, it is important to note that the enzymes are immobilized only at the surface of the sol gel (i.e., two dimensional), which is not buffered. Thus, the enzymes described in those disclosures have very limited stability, with a maximum activity lifetime of generally no more than 7 to 10 days.
Minteer et al. developed a biofuel cell, which includes an improved bioanode (disclosed in patent applications 60/429,829, 60/486,076 and Ser. No. 10/617,452), with an active life span of greater than 45 days with no degradation in performance. A particular embodiment of that biofuel cell used dehydrogenase enzymes and NAD+ as the anode catalyst and ethanol as the anode fuel, and an ELAT electrode comprising about 20% Pt on Vulcan XC-72 (E-Tek) as the cathode catalyst and dissolved O2 as the cathode fuel. The open circuit potential of that biofuel cell was 0.82 V at 20° C. and pH 7.15, and the maximum power density was 2.04 mW/cm2.
The improved bioanode includes a quaternary ammonium bromide salt-treated (modified) Nafion® membrane, which provides an ideal environment for stable enzyme immobilization. The modified Nafion® membrane, while retaining the electrical properties of unmodified Nafion®, was shown previously to have increased mass transport capacity for ions and neutral species, a lower acidity and a buffered near neutral pH than unmodified Nafion®, and an increased pore size to accommodate the immobilization of relatively large molecules such as enzymes (see Schrenk et al., 2002, J. Membr. Sci. 205:3-10, which is incorporated herein by reference).
Other biocathodes, which are less stable, less efficient and more toxic than the particular embodiment of the biocathode that is disclosed in this application, have been described in the literature (e.g., Chen et al., 2001). For example, the biocathode of Chen et al. utilizes a hydrogel membrane, which is not buffered and is only able to bind enzymes at the surface, the cathode enzyme laccase (EC 1.10.3.2), which has a pH optimum of 5 and is inactive in the presence of chloride ions, and an osmium complex as an electron transport mediator, which is toxic. There is a need for an improved biocathode including a cathode enzyme, which is not affected by chloride ions, a less toxic electron transport mediator, and a modified ion exchange membrane that incorporates the cathode enzyme within a buffered micelle.